Hi bro, in screen time we wont be able to know the battery usage. it calculates overall background usage of the app too. are you seeing any fast battery drain issue? whatsapp can still use notification services even if background app refresh is turned OFF. only it cannot use location services and other system services in the background that consumes more battery power.
can you check battery page graph of last 24 hours to see if whatsapp has more off screen usage?